(a) Field of the Invention
The present invention relates to a carbon steel sheet having high formability and a manufacturing method thereof. More particularly, the present invention relates to a carbon steel sheet having a microscopic and uniform carbide distribution, a fine grain of ferritic phase, and high formability, and a manufacturing method thereof.
(b) Description of the Related Art
Typical high carbon steel used for fabricating tools or vehicle parts is applied with a spheroidizing annealing process for transforming a pearlite texture to a spheroidized cementite, after it is produced in the form of a hot rolling steel sheet. A long period of annealing is required for complete spheroidizing. Accordingly, production cost increases and productivity is deteriorated.
In order to manufacture the hot rolling steel sheet, typical processes such as drawing, deforming, stretch flanging, and bending are typically applied to the high carbon steel for the fabrication after the hot rolling and winding and the spheroidizing annealing.
When the high carbon steel is made of a two phase structure including ferrite and cementite, the formability during fabricating the desired parts is significantly affected by the shapes, sizes, and distribution of the ferrite and the cementite. In the case of a high carbon steel having substantial amount of free ferrite texture, although it shows high ductility since carbide is not resident in the free ferrite, a stretch flange formability thereof (which can be graded by a hole expansion ratio) is not always excellent.
A texture of a high carbon steel having free ferrite and ferrite including spheroidized carbide includes the carbide in a larger size than that of the high carbon steel that only has the ferrite including carbide.
Therefore, holes expand during the fabrication process such that a deformation difference occurs between the free ferrite and the ferrite including the spheroidized carbide. In order to maintain continuity in the deformation of material, the deformation is concentrated on an interface between the relatively coarse carbide and the ferrite. Such a concentration of deformation causes generation of voids on the interface that can grow to a crack, and consequently stretch flange formability may be deteriorated.
When the steel having a texture of the ferrite and the pearlite is applied with the spheroidizing annealing, the spheroidizing annealing time is attempted to be reduced by processing a cold rolling after a hot rolling. In addition, when a gap in the lamellar structure of the carbide in the pearlite texture becomes narrower, i.e., when the texture becomes finer, the spheroidizing speed is improved such that the time for finishing the spheroidizing becomes shorter. However, in this case also, a batch annealing furnace (BAF) heat treatment is still required for a long time.
The high carbon steel for the fabrication is applied with a process for increasing the hardness such as a subsequent cooling process of quench hardening after an austenitation heat treatment. In this case, when the size and/or thickness of the material is small, the hardness may become uniform over the entire material. However, when the size and/or thickness of the material is not small, the hardness may easily become non-uniform. In many precision parts such as vehicle parts, a hardness deviation results in a deviation of durability. Therefore, obtaining uniformity of material distribution after the heat treatment is very important.
Methods for solving the problem of the non-uniform material distribution are found in Japanese Patent laid-open publication No. 11-269552, Japanese Patent laid-open publication No. 11-269553, U.S. Pat. No. 6,589,369, Japanese Patent laid-open publication No. 2003-13144, and Japanese Patent laid-open publication No. 2003-13145.
Firstly, according to Japanese Patent laid-open publication No. 11-269552 and Japanese Patent laid-open publication No. 11-269553, a hot rolling steel sheet having a free ferrite area ratio above 0.4×(1−[C]%/0.8)×100 and pearlite lamellar gap above 0.1 μm is fabricated from a metal texture of a substantially ferrite and pearlite texture, using steel having 0.1 to 0.8 wt % of carbon. Then, after processing cold rolling by more than 15%, a two step heating pattern is applied. Subsequently, the material is cooled and maintained at a predetermined temperature. Thus, a high or intermediate carbon steel sheet having high stretch flange formability is manufactured by applying three steps of heating patterns.
However, such a method is understood to have a drawback in that production cost increases since the cold rolling is performed before the spheroidizing annealing.
In addition, U.S. Pat. No. 6,589,369 discloses a method for fabricating steel plate having high stretch flange formability. C at 0.01 to 0.3 wt %, Si at 0.01 to 2 wt %, Mn at 0.05 to 3 wt %, P at less than 0.1 wt %, S at less than 0.01 wt %, and Al at 0.005 to 1 wt % are contained in the steel plate. Ferrite is used as a first phase. Martensite or residual austenite is used as a second phase. A quotient in a division of volume fraction of the second phase by average grain size is 3-12. A quotient in a division of an average hardness value of the second phase by an average hardness value of the ferrite is 1.5-7.
However, such a method cannot provide a high hardness value that is obtained by a cooling process after the austenitation heat treatment, which is an important factor in a typical high carbon steel. In addition, a uniform carbide distribution cannot be achieved when applying the spheroidizing heat treatment, and thus, the hole expansion ratio is deteriorated after final spheroidizing.
According to Japanese Patent laid-open publication No. 2003-13144 and Japanese Patent laid-open publication No. 2003-13145, a hot rolled or a cold rolled carbon steel sheet having a high stretch flange formability is produced. In the method, a hot rolled carbon steel sheet is fabricated by hot rolling a C-steel of 0.2 to 0.7 wt % at a temperature above Ar3-20° C., cooling at a cooling speed of more than 120° C./second, stopping the cooling at a temperature above 650° C., subsequently cooling at a temperature below 600° C., applying pickling, and then annealing at a temperature of 640° C. to Ac1 temperature after pickling. The cold rolled carbon steel sheet is fabricated by application of cold rolling of above 30% after the pickling of the hot rolling steed sheet, and then annealing at a temperature of 600° C. to Ac1 temperature.
According to the above method, the cooling at the cooling speed of more than 120° C./second after the hot rolling is not possible in a typical hot rolling factory, and thus a cooling apparatus that is specially designed for that purpose is required, which causes a drawback of high cost.